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The loss of the heavy water was absolutely decisive. It stopped their reactor programme in its tracks.

Professor Eric Grove

The Nazis launched their nuclear programme in April 1939, and took control of the key plant at Vemork, which produced heavy water near the lake, when they invaded Norway in 1940.

The Allies, fearing Hitler would build a nuclear bomb to obliterate London, ordered a series of daring raids to destroy the plant.

In 1943, Norwegian commandos blew up the plant, with the raid later immortalised in the Kirk Douglas film The Heroes of Telemark.

Following the raids, the Nazis started moving barrels of the remaining heavy water from Vemork to Germany by ferry and train.

Churchill ordered series of daring commando raids on the Norwegian plant being used by the Nazis (Image: GETTY)

Allies blew up the SF Hyrdro ferry used to transport heavy water atomic bomb material in 1944 (Image: Wikipedia)

But Norwegian resistance fighters blew up the ferry using a timed bomb, exploding it when it reached the middle of the lake.

Technology used by the National Geographic team shows the boat still in one piece at the bottom of the lake.

Dr Fredrik Soreide, of the Norwegian University of Science and Technology, said: “We wanted to go down and take up a barrel to prove that this was in fact the heavy water that was being shipped to Germany.”

The 10-part series Drain the Oceans starts on Thursday, September 6 on the National Geographic channel.